A central goal of flexographic printing is to produce the best possible print quality, most economically. In today’s brand-conscious market, high print quality is of paramount importance in attracting customers and maintaining a loyal customer base.
Brand owners require the highest possible color accuracy for packaging, as brand identity is closely linked to color accuracy. Consider Coca-Cola red or Heineken green—these are emblematic for a whole range of global products. In today’s world of rapidly changing and diversifying packaging for similar products, it is essential this color accuracy and consistency be maintained despite frequent and necessarily fast setup job changes.
Although much has been written about objective criteria for color quality, most boils down to reducing the Delta E value to less than 2, which is a generally accepted criterion for job approval. This approach has two shortcomings1:
- First of all, there is no direct path for translating spectrophotometer measurements to specific actions that either the operator or the machine can execute to correct a too-high Delta E value
- Perhaps the more serious problem is that the Delta E value is not the ultimate judge of the print’s quality—it is the customer who finally decides whether or not the color is acceptable. A satisfied customer ensures better economy through less returned waste and promotes long-term business relationships through repeat orders
Rheonics’ InkSight Viscosity Control System, a 2021 FTA Technical Innovation Award recipient in the prepress/pressroom category, was developed in cooperation with one of this article’s authors, Bert Verweel, who had tried a variety of viscosity sensors and control systems and was dissatisfied with the results2. Among the most common shortcomings were lack of stability, reliance on cup measurements for frequent and necessary calibration, and sensitivity to contamination by ink deposits.
The Rheonics SRV viscometer, a key component in the InkSight system, answered the need for a stable, accurate and robust sensor; the InkSight was developed to support the goal of color excellence—beyond Delta E 2000—to enable the highest, most consistent color quality, both initially during job setup and throughout the longest runs, by tight, accurate and reliable viscosity control.
In this article, we will show how and why accurate control of ink viscosity is one of the most important factors contributing to the final quality of the printed image. We will start by offering experimental evidence for the link between ink viscosity and Delta E 2000 values, and how these experiments enable us to quantify the permitted viscosity variation for a given limiting value of Delta E 2000.
From this information, we can evaluate the advantages and limitations of available ink viscosity measurement systems. This will permit us to demonstrate how the measurement and control technology that forms the core of the InkSight system enables tight control of Delta E 2000 values on even the longest printruns, while minimizing setup times and operator intervention.
Color & Viscosity Control
Delta E 2000 can be measured, but not controlled—In conventional flexographic printing, there is no direct way of correcting Delta E 2000, either by hand or by a control system. Therefore, the operator must have a method for translating Delta E 2000 values—as well as deviations from color master on visual inspection—into concrete actions that can improve and maintain good color matching.
Delta E 2000 can be directly linked to viscosity through color density—There is an established link between Delta E values and other printing parameters1. Once the operator knows how the required inks behave on the given substrate and the flexographic machine, color density emerges as the most important predictor of color accuracy. And color density, in contrast to Delta E value, is controllable by clear and explicit actions that are available to the operator.
Color density is dependent of the way that the ink film is deposited and dries on the substrate. And against intuition, more is not necessarily better. This is demonstrated by an experiment that is described later in this article.
Viscosity Control Is the Key
Bert Verweel, one of the authors of this article, conducted a series of experiments to explore the potential of viscosity measurement and control as a means for optimizing color quality. While Delta E 2000 was one of the criteria for judging the results, visual inspection proved to be a far more sensitive test for color quality and showed the true value of accurate viscosity control.
We performed a series of experiments on the effects of ink dilution on print quality and measured ink viscosity. We will see how this then allows estimating the required accuracy of viscosity measurement in order to maintain Delta E 2000 < 2 for an entire run. In a first experiment, 10-kg. ink was diluted by 10 percent. The press was run at 200 meters/min, the polyester film was marked and the press was stopped. Ink was diluted with a further 3 percent of solvent, then circulated until the viscosity stabilized and the process was repeated a total of 15 times. The film was removed, and all 15 segments were measured with a spectrophotometer, and photographs of the film segments were made for subjective visual evaluation. Figure 1 shows the visual appearance of the printing quality at a series of dilutions.
At the lowest dilution (highest viscosity), too much ink is deposited and does not flow properly. Pinholes develop and overall quality is poor. Although the color between the pinholes is quite dense, the measured density is low, due to the high reflectivity of the pinholes. As dilution increases, viscosity decreases and the flow improves, but pigment loading decreases and the color becomes lighter. Each sample was measured with a spectrophotometer and compared with the digital PMS reference. Figure 2 shows the Delta E 2000 as a function of viscosity. The Delta E 2000 values are referred to a digital PMS reference.
On one end of the scale, the pigment loading is insufficient to produce the desired color density; on the other end, the viscosity is too high, leading to poor flow and pinholes in the ink layer. Between these two extremes is an optimum viscosity that produces both the optimal pigment loading flow characteristics, and therefore the desired color quality. What determines the pigment loading and viscosity of an ink? There are two contributing factors:
- The first is the ink’s initial composition, the “recipe” that is used to prepare the ink as delivered by the supplier to make it ready for the printrun
- The second factor is degree of dilution—the amount of solvent added to the ink, both in the initial preparation and in later additions to keep the pigment loading and flow properties constant throughout the printrun
Both of these factors have a direct link to an easily measurable and controllable quantity: ink viscosity.